1. Field of the Invention
The present invention relates to a method and apparatus for the formation of a continuous web of a tubular thermoplastic film from an extrudate of a thermoplastic polymeric material or resin and, more particularly, relates to the formation of a thermoplastic film having a profiled wall contour extending in the direction of the longitudinal axis of the film.
The formation of thermoplastic tubular film through the extrusion of molten thermoplastic polymeric materials wherein the materials are conducted through an annular extrusion die orifice, employed for the manufacture of film, tubing, pipes and the like, is well known in the plastics extrusion technology. In order to regulate the rate of cooling and solidification of the extrudate, it has frequently been found desirable to contact the surface of the molten or still heat-plastified thermoplastic material, upon leaving the extrusion die orifice, with a stream of a suitable cooling medium and, as a result, to control the properties and physical configuration of the finished extruded product.
Various commercial products, for example, such as the extensively sold sandwich bags which are formed from a thermoplastic film material, require material thicknesses which must be heavier towards the top or opening of the bag where they are normally gripped and handled by a user in order to minimize tearing of the film and to concurrently make the bag feel thick and of high strength and quality to a user. It is not necessary to provide a heavy gauge of film in the body of the bag where it is rarely handled and wherein a very low mechanical stress is developed. Consequently, a concept has been developed which is designed to effect substantial savings in the required quantities of polymeric material or resin without sacrificing performance of the finished product by profiling the film thickness upon extrusion of the polymeric resin or material so as to provide the desired film thickness at the top of the bag while allowing for a thinner film thickness in the body of the bag, so that the average film thickness is, in fact, much thinner than that which is felt and observed by the user of the bag.
2. Discussion of the Prior Art
Heretofore, various kinds of methods and apparatus have been proposed with regard to manufacturing techniques in the formation of thermoplastic film possessing profiled thicknesses; nevertheless, none of these have proven themselves in actual practice to be completely satisfactory in affording the desired degree of control in the formation of extruded profiled thermoplastic film.
Behr U.S. Pat. No. 3,852,386 discloses the production of an extruded plastic tubular film having circumferentially spaced thickened film sections or profiles extending continuously in the axial direction of the film. These sections are adapted to interlock mechanically with each other so as to constitute a zipper-like fastener integrally formed with the film. The profiled sections in the film are formed through the provision of different rates of cooling after the film has been extruded from an annular die subsequent to the expansion of the tubular film, but while the film is in a still hot state. As may be clearly ascertained, the formation of the profiled sections in the film wall is effected in a region having been subjected to primary cooling and inflating of the film, in which such profiling is rather difficult to achieve in any satisfactorily controlled manner. Consequently, the apparatus and method disclosed in this patent is bounded by extensive restrictions in the formation of precise profiled wall sections in extruded thermoplastic films.
Similarly, interrelated British Patent Nos. 1,250,945 and 1,236,831 each disclose processes for the production of tubular thermoplastic film which includes at least one longitudinal zone wherein the wall thickness of the film is heavier than the thickness of the remainder of the film, and also describes the formation of bags with that process. In each of the British patents, the selective cooling of the extruded film is effected subsequent to the primary cooling and inflating of the tubular plastic film, which renders the formation of controlled profiled wall structure extensively difficult, if not completely impossible.
More recently an apparatus and method has been developed, as disclosed in Herrington U.S. Pat. No. 4,209,475, which describes an apparatus for adjusting a cooling medium air flow from a cooling ring encompassing an extrusion die orifice so as to cool more extensively in selected regions of the film. The patented apparatus provides for a large number of adjustable deflector blades which are circumferentially arranged about the extrusion die orifice of the apparatus, and are basically designed to provide adjustment so as to compensate for errors or irregularities in the thickness of the plastic film produced by the extrusion die. Conceivably, the apparatus can be employed in producing a variation in the film thickness when the extrusion die is of a precise configuration and, upon effecting displacement of the deflector blades, is capable of producing extruded thermoplastic tubular film with a profiled thickness. However, the apparatus operates with relatively high air cooling speeds and flow rates and is not designed to provide a controlled profiled tubular film in a manner utilizing relatively low air cooling flow speeds and rates as is contemplated by the inventive method and apparatus.
Basically, in accordance with the inventive concept, when forming extruded tubular thermoplastic film which is intended to be utilized in the formation of sandwich bag film, the cooling speed or rate is considerably lower than that employed with usual tubular plastic film extrusion processes, so that the amount of cooling air which leaves the air ring is relatively small. Consequently, there is necessitated the application of a different process and apparatus which will provide for the selective or localized cooling of the extruded tubular plastic film at a location in which the thermoplastic polymeric material or resin in the primary stage of forming the film is still in a molten or heat-plastified condition, and in which a separate flow of a cooling medium is directed against the extrudate at specific regions or zones about the periphery of the tubular film material being extruded so as to form one or more profiled film regions.